Photosensitive apparatus



Sept. 19, 1950 R. c. MASON EIAL PHOTOSENSITIVE APPARATUS Filed Aug. 10, 1949 .Opflcal Magnifier INVENTORS Ruric 0. Mason 8 ATTORNEY v of matter.

Patented Sept. 19, 195

PHOTOSENSITIVE APPARATUS Ruric 0. Mason and John W. Coltman, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 10, 1949, Serial No. 109,585

Claims.

Our invention relates to photosensitive a pparatus, and it has particular relation to electron optical amplifiers for images produced by transmitting radiation of the X-ray type through objects. This application relates to applications Serial No. 771,112, filed August 28, 1947, to Lloyd PrHunter and Richard L. Longini, and Serial No. 771,113, filed August 28, 1947, to Richard L. Longini.

By radiation of the X-ray type, we mean high frequency or :particle radiation of any type which can be transmitted through a body to produce images of its internal structure including ordinary X-rays produced by medical or industrial X-ray tubes or the radiation produced by radioactive materials of the various types now available. In the following discussion we shall refer to such electron optical amplifiers as X-ray image amplifiers or imageamplifiers.

X-ray. image amplifiers constructed in accordance with the teachings of the prior art of which we are aware include an X-ray responsive fluorescent screen. The radiation transmitted by the object under observation is projected on this screen and produces thereon an optical image. The light emitted from this screen impinges on a photoelectric surface causing the latter to emit electrons. Anelectron image corresponding to the optical image is thus produced. The electron image is focused by electron optical means on an electron-responsive screen producing an optical image thereon. It is customary torefer to the X-ray responsive fluorescent screen as the input screen and the electron-responsive screen as the output screen.

An image amplifier of the type just described has proved inadequate. The usual purpose of such an amplifier is to view in detail internal structure which is buried under the large mass For example, medical radiology is concerned with the examination of the intestinal tract and other portions of the lower abdomen. In producing X-ray images of these regions of the body, the radiation is transmitted through substantial thickness of human tissue, which obscures the image produced. Prior art apparatus does not avail a satisfactory image.v

It is, accordingly, an object of our invention to provide an image amplifier which shall produce a bright, well-defined image.

Another object of our invention is to provide an image amplifier which shall produce a bright,

well-defined image of internal structure of geometrically complex objects having substantial thickness.

A more specific object of our invention is to provide an image amplifier which shall produce well-defined images of small components of structures of the human body which are buried beneath thick layers of tissue, such as, for example, the intestinal tract or the bone structure at the base of the spine.

A general object of our invention is to provide an amplifier for images produced by radiation of the X-ray type which shall have high brightness and high resolving power.

, An ancillary object of our invention is to provide an improved image tube.

Our invention arises from the realization that the obscurity of the image produced with prior art image amplifiers may be ascribed to the fact that the signal-to-noise ratio of the electron image emitted by the photoelectric surface is not sufficiently high for the desired image clarity. We have further realized that the background noise which obscures the desired image arises principally from thethermal electron emission of the photoelectric surface.

The electrons of any body, such as a photoelectric surface, are held in the body by the forces which bind them to the atoms of the body. These electrons also have a random motion which is imparted to them by the thermal energy which the body receives from its surroundings. The temperature of the body is a measure of the average of this energy of random motion. At any temperature, certain of the electrons have sufficient energy to overcome the forces which bind them to the atoms of the body and these escape from the surface. The number of such electrons which escape at any temperature is roughly inversely dependent on the magnitude of the forces which bind the electrons to the atoms. It is these electrons which are manifested as noise in the display of an electron image and the greater this number under any condition defined by a set of parameters, the greater the noise and the smaller the signal to noise ratio.

The forces which the thermal electrons overcome in leaving the body are measured by the so-called thermionic work function. The electrons which are ejected from a photoelectric body by light quanta are held by forces which are measured by a so-called photoelectric work function. The latter function is for all materials either equal to or greater than the thermionic Work function.

In conceiving our invention, We have realized that a photoelectric surface, which has a low photoelectric work function must necessarily have a low thermionic work function and, therefore, must emit thermal electrons substantially. The photoelectric work function is measured by the frequency of the lowest frequency photons which eject electrons from the body; that is, by the threshold frequency, 110. We have realized that if we provide an image amplifier having a photoelectric surface for which the magnitude of 110 is substantially higher than for other surfaces, such a surface may have a lower thermal emission than the other surfaces and in actual fact we found that certain surfaces having a high 110 do have a low thermal emission.

In image amplifiers constructed in accordance with the teachings of the prior art, the threshold frequency of the photoelectric surface is in the infra-red range (11,000 Angstroms wavelength) and has a maximum response in the red-near-infra-red range, that is, 1/0 is relatively low. Such a surface inherently could not have a low thermal emission.

In accordance with one aspect of our invention, we provide an image amplifier having a photoelectric surface which has a threshold frequency near the center of the visible spectrum (approximately 6,000 Angstroms wavelength). We have found that the improvement which is thus effected in the reduction of thermal noise in our image amplifier is enormous.

Another and more specific aspect of our invention arises from our realization that to the extent practicable the radiant energy emitted from the object under observation should be conserved. This condition is imposed b the inherent character of this radiation.

The radiation transmitted through the object under observation does not flow in a homogeneous stream. It is made up of a large number of individual bundles or quanta of energy. As each quantum impinges on the input fluorescent screen, it produces a scintillation of light. Thus this image is in fact formed from a large number of such scintillations and the detail which can be observed is ultimately limited by the number of these scintillations per unit area per unit time. In the medical art, the number of input quanta to the input screen is limited by the necessity of protecting the patient from injury by reason of excessive radiation. At the present state of the medical art, the input radiation is already at its maximum limit, a limit which from the standpoint of the number of scintillations available per unit area per unit time is relatively low. We have realized that if a substantial proportion of the initial quanta, or the initial scintillations, are lost in the transforma' tions through which the radiant energy passes, the output optical image will be made up of a large number of flickering light spots spaced by relatively large distances and the image will be so obscure as to be uninterpretable.

In accordance with our invention, we provide a combination of a photoelectric surface and a fluorescent screen such that each quantum of .X radiation absorbed in the fluorescent screen produces a large number of photons which eventually produce a number of photoelectrons that is large compared to one. If this condition is satisfied, we will assure that each original quanergy over the binding energy (him).

electrons (that is, than hue).

tum absorbed contributes in forming the final image.

Accordingly, the photoelectric surface which we provide in accordance with our invention has a high sensitivity and the input fluorescent screen which we provide is so matched to this surface that the two combine to conserve the input quanta to the extent practicable. The input fluorescent screen has the property that substantially each X-ray quantum transmitted through the object under observation and absorbed in the screen causes the emission of a very large number of light photons. The input screen in its emissive properties matches the photoelectric surface so that a substantial fraction of the photons emitted by the input screen causes the ejection of a photoelectron from the photoelectric surface. In the amplifier in accordance with our invention in which the photoelectric surface is responsive predominantly to light in the blue-near-ultra-violet, the fluorescent screen is designed to emit light substantially only in the blue and near ultra-violet.

A still further aspect of our invention arises from our realization that an additional cause of the obscurity of the ultimate optical image is the aberration which exists in the electron optical lens system of the prior art. This aberration arises from differences in the velocity of the electrons which constitute the electron image. The energy impressed by the photons on the photoelectric surface, should be somewhat greater than the threshold energy of the photo- When an atom absorbs this energy and emits an-electron, the energy with which the electron leaves the photoelectric surface is roughly dependent on, and must be less than, the excess of the photon en- The velocity of an electron is proportional to the square root of its energy.

The photo-electrons are emitted from the surface at different angles with reference to the direction of the electric field into which they are energy (light energy from the input screen) is large compared to the threshold energy (hl/O) the electrons are ejected at high velocities in the different directions and their velocities in the direction of the field vary over a wide range. Because the electrons under the assumed circumstances have large velocity components in the direction perpendicular to the direction of the field and vary in velocity overa wide range in the direction of the field, they tend to spread over a wide beam. This variation exists to an extent in situations in which the impinging photon energy is monochromatic. In'situations in which the light from the input screen has frequencies extending over a wide band, this undesirable condition is accentuated.

We have realized that if the excess in the photon energy over the threshold energy is substantial, substantial aberration is produced by the resulting wide variation in the velocity of the photo-electrons in the electron optical lens system. Aberration would be minimized if the photon energy were distributed over a narrow band of a frequency just higher thanthe threshold frequency. However, in the immediate region of the threshold frequency, the response of the photoelectric surface is relatively small and few tical magnifying lens electronswould be emitted in response to the photons. We have accordingly also realized, that photoelectrons.

In accordance with a further aspect of our invention, therefore, theinput fluorescent screen is such that'thelight which it emits has a frequency which extends only over a relatively narrow band. The extent and position of this band on an energy scale shouldbe the minimum compatible with reasonable. efiiciency. The frequency band is such that the photon energy is somewhat greater than the energy binding the electrons of i the photoelectric surface to the atoms.

We have found that the photoelectric surfaces which have a threshold near the center of the visible spectrum (6,000 Angstroms) and which are used in accordance with an aspect of our invention usually have their maximiun response in the blue ultra-violet portion of the spectrum. Such a surface yields satisfactory results from an electron. optical aberration and number of electrons standpoint when combined with input screens which emit light in the blue and ultradunderstood from the following description of a specific embodiment when read in connection with the accompanying drawing, in which:

.Figure 1 is'a diagram illustrating our invention, V i i Fig. 2 is a. view in section of an image tube in accordance with our invention, 7

Fig. 3 is an enlarged view in section of the composite input plate of the tube shown in Fig. 2, and .9

Fig. 4 Ban enlarged view-in section of the compositeoutput plate of the tube shown in Fig. 2.

The apparatus shown in Fig. 1 includes an X- ray tube 5,. the radiation from which is projected through an object 1 under observation onto the input plate 9 ofan image tube II. The output plate I3 of the tube I I is observed through an op- I5. The X-rays pass through the object 1 under! observation and after impinging on the input plate 9, are converted into an optical image which isviewed through the lens [5.

While we have shown in Fig.1 an X-ray tube 5, we do not intend that our invention be limited to such a tube. The tube may, for example, be replaced by a material such as radium which emits penetrating gamma rays. While we have shown the object 1 under observation as a perj son, manifestly a doctors patient, our invention is applicable to the study of inanimate objects,

such as, for example, the components of motors or generators, metal tubes, or metal strips of any thicknesses.

I The image tube and the principal components of its circuit are shown in detail in Figs. 2 to 4. The tube'comprises an evacuated envelope I! which has a transparent end IS in the form of a spherical sector. Adjacent this end, the composite input plate 9 is mounted.

This plate includes an aluminum sheet 2| in the form of a spherical sector on the concave surface of which a thin layer 23 of a mixture of 6 fluorescent material and powdered glass is de posited. This layer which constitutes the input screen includes a suitable phosphor. It should have the property of emitting light in the higher frequencies of the visible and ultra-violet portion of the spectrum. In particular, I prefera phosphor which emits light of a narrow band of ,frequencies in the blue and near ultra-violet por-i tions of the spectrum. The phosphor used in accordance with the preferred practice of our invention is identified as No. 1101 by its manufacturers, the Patterson Screen Division of du Pont De Nemours Company, which-is located at To-. wanda,Pe nnsylvania. It is described as a zincsulphide silver. activated X-ray phosphor. This phosphor is mixed with a powdered glass of the type defined in application Serial No. 101,963, filed July 29, 1949, to Walter J. Hushley, and is deposited on the aluminum sheet in the manner outlined in application Serial No. 101,964, filed June 29, 1949, to Walter J. Hushley.

We have found that other phosphors are also to an extent satisfactory. For example, zincca-dmium-sulphide phosphor, barium leadsulfate phosphor and other phosphors areto an extent satisfactory. The phosphor disclosed in the above cited application Serial No. 771,113 to Longini could also be used. The use of such other phosphorsare within the broader scope of our invention.

On the input fluorescent scren 23, a transparent conductive layer 25 is deposited. A suitable transparent conductor is manufactured by I the Pittsburgh Plate Glass Company and is identified by the trade-name Nesa.

A photosensitive surface 21 is deposited on the transparent conductive layer 25. This. photosensitive surface should have the property of emitting electrons substantially only when light of the frequencies. equal to or greater than that of the light emitted by the input fluorescent screen impinge thereon. If the input screen emits light in. the blue and near ultra-violet, the photosensitive surface should be responsive predominantly to light in the blue and near ultraviolet; there may be additional response to light of higher frequencies. In the preferred practice of our invention, the photosensitive screen is composed of antimony on which a caesium layer has been deposited by flashing in a vacuum and which after such deposit has been heated to a temperature between 150 C. and 210 C. to form a caesium antimony compound (CSxSb; "probably a mixture of Cs2Sb and CsaSb). In the following specification we shall refer to this photoelectric surface as of the caesium antimony type.

We have found that surfaces. produced by flashing caesium on arsenic or bismuth or any mixture of arsenic, antimony or bismuth and heating is also satisfactory. We have also found. that surfaces produced by flashing other metals of the alkali group, particularly rubidium, or mixtures of these metalson deposits of arsenic, antimony or bismuth or mixtures of these is also sat.-

ciently conductive; In such situations, the transparent layer 25 may be non-conductive, for example, it may be glass of the type with which the fluorescent material of the input screen 23 is mixed. I

The caesium antimony surface 21 has a sensitivity in the blue near ultra-violet range which is high compared to the photoelectric surfaces of image amplifiers of the prior art. We have found that in the frequency region of emission of the phosphor 23, the surface 21 has a sensitivity of the order of .03 ampere per watt, whereas the prior art surfaces have a sensitivity in the region of the emission of the input screens with which they are associated of the order of .0005. That is, the prior art photoelectric surfaces would produce electron images which contain of the electrons of our electron images for equal brightness of input screen. In actual fact, the prior art screen has a brightness of the order of the brightness of our preferred screen (23) for the same input intensity. Accordingly, our electron image has approximately 180 times the electrons of the prior art images. With input screens including the other phosphors mentioned above and photoelectric surfaces composed of the elements or combination of elements of chemical groups I and V mentioned above improvements approaching the above described improvement of 180 are achieved.

A plurality of. conductive cylinders 29, 3!, 33, 35 and 31 of progressively increasing length and progressively decreasing diameter extend from theinput plate 9 longitudinally along the envelope IT. The first of these cylinders 29 is insulated from the input plate 9 and each of the other cylinders is insulated from succeeding and preceding cylinders. Potentials of different magnitudes are impressed on the cylinders 29 to- 31 from a suitable voltage divider 39. The cylinders function as an electron optical lens system for the electron image emitted by the photosensitive surface 21.

The cylinder 31 having the smallest diameter extends into a constricted neck 4| of the envelope l'l. At the terminal of this neck, very near to the opening of the cylinder 31, of smallest diame ter, the composite output fluorescent plate I3 is -mounted. This composite plate is composed of an electron-responsive phosphor screen 45 on which-a layer 43 of aluminum is deposited. The phosphor may be of any suitable type but in accordance with the preferred practice of our invention is composed of zinc cadmium sulphide and is identified by RCA Victor, its manufacturer, as 33Z604B. The aluminum layer is of sufiicient thickness to obstruct back emission of light from the output phosphor to the photosensitive surface 2! and, sufiiciently thin to permit penetration of the electrons.

While the tube shown in Fig. 2 is of the type having an electrostatic electron optical lens system 29 to 39, our invention is not limited to such a system. A tube having a magnetic lens or a combined electrostatic and magnetic lens is within the scope of our invention.

The thermal-emissivit of the photoelectric surface 21 is relatively low because the electrons of the surface are relatively strongly bound to the atoms. For this reason, the background noise of the electronic image emitted by the surface is low. The input fluorescent screen 23 is designed to emit light in the frequency range to which the photoelectric surface 2'! is predominantly responsive. It is also designed toemit large numbers of hotons for each of the X-ray quanta impinging thereon. The X-ray energy is thus to a large extent conserved and flickering is avoided. The radiation-emitted by the input fluorescent screen 29 is moreover limited to a narrow frequency range centered at a point near the threshold frequency. For this reason electron optical aberration is minimized with the apparatus disclosed herein, and a bright image clearly defining the various components of the part under observation is produced.

While we have shown and described a certain specific embodiment of our invention, many modifications thereof are possible. Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

We claim as our invention:

1. An image amplifier tube characterized by an X-ray responsive fluorescent screen of the zinc sulphide type and by a photoelectric surface which responds substantially only to light in the blue, ultra-violet and higher frequencies.

2. An image amplifier tube characterized by an X-ray responsive fluorescent screen which when radiation of X-ray type impinges thereon emits light predominantly of light frequencies in the blue, ultra-violet and higher frequencies and by a photoelectric surface of the caesium antimony type.

3. An image amplifier tube characterized by an X-ray responsive fluorescent screen of the zinc sulphide type and by a photoelectric surface of the caesium antimony type.

4. An image amplifier tube characterized by an X-ray responsive fluorescent screen which when radiation of X-ray type impinges thereon emits light substantially only in a narrow band extending between the blue and the near ultraviolet and by a photoelectric surface which responds strongly to light in said band and has a threshold frequency in the central portion of the visible spectrum.

5. An image amplifier tube characterized by an X-ray responsive fluorescent screen which when radiation of X-ray type impinges thereon emits light substantially only in a narrow band extending between the blue and the near ultraviolet and by a photoelectric surface which has a threshold frequency in the central portion of the visible spectrum.

RURIC c. MASON. JOHN w. COLTMAN.

REFERENCES CITED The following references are of record in the file of, this patent:

UNITED STATES PATENTS 

